The basic structure of a primary alkaline electrochemical cell is known. Generally speaking, alkaline cells include a positive electrode ("cathode") that receives electrons from a negative electrode ("anode") that releases electrons. The cathode is joined to a positive terminal of the battery by a collector rod. The negative electrode is typically a high-surface area metal such as zinc. The anode metal is provided in an electrolyte solute, such as potassium hydroxide, which is the ion transfer medium between the anode and the cathode. A separator which passes ions, but not electrons, is placed between the electrodes. Other aspects of a typical alkaline cell are described elsewhere in the specification.
It is common in the art to provide a gelled anode wherein the gelled portion includes the anode metal, provided as a powder, an aqueous alkaline electrolyte, and a gelling agent for fixing the anode metal and electrolyte in the gel state. Conventional gelling agents include carboxymethylcellulose, crosslinking-type branched polyacrylic acid, natural gum, or the like. A typical anode metal is zinc powder.
When formulating a gelled anode, it is important to ensure that the anode remains dispersed in the gel and that the gel retains its integrity. If dispersion is reduced or if the gel experiences syneresis, the effective surface area of the anode is reduced and the anode network required for efficient ion transfer is interrupted. Contact among the particles of an anode network, or between the anode particles and the cathode current collector, can also be reduced or interrupted when an alkaline cell is dropped, jostled, or vibrated. This shock sensitivity is a particularly well known problem of mercury-free alkaline cells employing crosslinking-type gelling agents. Shock sensitivity can result in a high internal resistance, a rapid decrease in cell voltage, and other problems, all of which are unacceptable to manufacturer and consumer alike. Problems resulting from erratic electrical conductivity between anode and negative current collector have been matters of particularly great concern since the industry reduced or eliminated mercury from the anode mix used in primary alkaline cells.
Another problem that can affect alkaline cells is that the zinc and the electrolyte can separate, concentrating the zinc and reducing the amount of electrolyte available for the anodic reaction at the zinc surfaces. One method for preventing this problem is to increase the viscosity of the gelled anode. However, there are practical limits on raising the viscosity, as discussed below.
As one solution to the problems associated with shock sensitivity, U.S. Pat. No. 4,963,447 (Nishimura) describes a gelled zinc anode containing a granular crosslinking-type branched polyacrylic or polymethacrylic acid gelling agent having a main particle diameter of 200 to 900 microns which concentrates the zinc into a reduced volume. According to U.S. Pat. No. 4,963,447, the gelling agent particles are crosslinked in the presence of a mixture of a polyvalent allyl crosslinking agent with a polyvalent vinyl crosslinking agent which are then granulated during or after deposition-polymerization. U.S. Pat. No. 4,963,447 carefully recites the importance of both the particular combination of crosslinking agents (col. 3) and of the polymerization method (cols. 1 and 2). Gelling agents of comparable size obtained after mass polymerization, suspension polymerization, or emulsion polymerization are described therein to be inadequate as gelling agents because they fail to contain a sufficient volume of electrolyte and because the electrolyte is insufficiently utilized.
Although the polyacrylic and polymethacrylic acid large particle gelling agents of U.S. Pat. No. 4,963,447 were an improvement over prior gelling agents, it is now recognized that inability to control the nature of the particles results in undesired properties in gelled anodes of alkaline cells. In particular, such gelling agents increase the viscosity of the gelled anode to a high level. High viscosity materials are disfavored in battery production processes because they make it difficult to regulate the amount of the gelled anode loaded into the anodic cavity of the alkaline cell. However, at suitable lower viscosities, detrimental effects are noted, such as a zinc-electrolyte separation, decreased zinc network robustness and decreased electrical discharge performance.
Loading of a high viscosity gelled anode into the anodic cavity of a cell at high applied stress rates is easier if the gelled anode is a non-Newtonian fluid whose viscosity decreases as the rate of applied stress increases. Although a gelled anode seems stiff when at rest, it liquifies and flows easily when a stress is applied at a high rate. This concept, referred to as "shear thinning," was applied to battery anodes by Meltzer and Krebs in U.S. Pat. No. 3,207,633. As was noted by Meltzer and Krebs, the effect of shear thinning is increased, and handling during alkaline cell manufacture is improved at lower viscosities.
The manner in which the gelling agent acts to push zinc particles into spaces among its swollen particles to promote contact among the zinc particles, or between the zinc particles and the negative electrode current collector, is understood. Use of gelled anodes has been shown to improve electricity production, but existing batteries are still subject to loss of contact when dropped or vibrated. There is still considerable room for improvement in both of these areas, particularly in view of the desire to further eliminate toxic materials from primary cells. Such toxic materials as mercury had previously been used to improve cell performance and reduce shock sensitivity.
The industry has also seen an increased demand for use of such cells in high-current environments, including portable audio equipment and cameras and flashes where cells are likely to discharge faster than in previous applications and are more sensitive to erratic internal resistance.
Thus, a suitable gelling agent capable of maintaining better electrical conductivity in a gelled anode while having an acceptable viscosity lower than that previously believed necessary to maintain a superior zinc network and offer improved electrical discharge performance is highly desirable. It is also desirable that a longer battery life be achieved in combination with the other desired advantages.